For many years, it has been recognized that metals, plastics and other medium and low strength materials could be reinforced with silicon carbide whiskers. These whiskers are small fibers, which can be mixed with the powdered metal or plastic to form a composite material, or which can be coated upon a thin sheet of metal, plastic or ceramic.
The silicon carbide whiskers are obtained by well-known processes, such as by reacting coked rice hulls, a throw-away byproduct of rice production, as is taught in U.S. Pat. No. 3,754,076 issued Aug. 21, 1973 to Cutler.
The whiskers, being fibers with anisotropic properties, are used to greatly reinforce and improve the mechanical properties of other materials.
While the advantages of utilizing these whiskers are well-known, it has been very difficult to properly mix these whiskers with other substances due to their initial aggolmerated state. The mass of agglomerated whiskers when mixed with other materials will produce a non-homogeneous composite of lower mechanical strength.
In order to achieve the maximum benefits of whisker reinforcement, it is necessary to thoroughly and uniformly disperse and/or distribute the whiskers. Such dispersion requires thorough deagglomeration and mixing without substantial damage to the whiskers.
It has been taught in the prior art that mixing whiskers with matrix material and a non-polar solvent containing a polymeric binder can provide a random distribution of whiskers in the matrix. Such a teaching is shown in U.S. Pat. No. 4,259,112, issued Mar. 30, 1981.
While U.S. Pat. No. 4,259,112 asserts being able to achieve a homogeneous composite, this homogeneity is not free of agglomerates as is the present invention.
Expressed in another way, the U.S. Pat. No. 4,259,112 does not teach deagglomerating the whisker mass. These agglomerates can seriously detract from mechanical performance.
In patent application U.S. Ser. No. 274,256, filed June 16, 1981, now U.S. Pat. No. 4,463,058, patented July 31, 1984, it is taught that the agglomerated whisker mass can be deagglomerated prior to mixing by treating it with a polar solvent and milling the solvated mass into a deagglomerated slurry. Alcohols, water, ketones and other organic solvents are good polar solvents for this purpose. These directionally charged solvents are believed to work by reason of their ability to neutralize the statically charged whiskers of the agglomerated mass. This neutralization allows the whiskers to separate, i.e. deagglomerate, upon subsequent milling of the slurry.
It has been discovered by the present inventors that a second slurry of previously deagglomerated whiskers will provide a further dispersion of the whiskers.
When this second slurry of deagglomerated whiskers is then coated upon a sheet, and the slurry is dried, a highly homogeneous, uniformly dispersed whisker and sheet composite is achieved, which is substantially free of agglomerates. The whiskers will adhere to the sheet by static and/or molecular attraction.
The above deagglomerating process has produced aluminum sheet composites of superior uniformity. This is the first time to the best of our knowledge and belief, anyone has been able to consistently achieve a uniform aluminum sheet and silicon carbide whisker composite.